A simple 1750Hz toneburst circuit, suitable
for incorporation into a typical ex-PMR rig. The one shown is a
simple one-IC design, It's based on a CMOS 4060 divider
IC, which has its own built-in inverter gate which can be used
as a simple oscillator.
A commonly available and low-cost 455KHz ceramic resonator
is used for this - the resonator is available from Maplin. It's
pulled down in oscillation frequency from 455KHz slightly by
the two 1nF capacitors connected to ground. The divide-by-
256 line output is used from the IC, which provides a 1750Hz
square wave, ie composed of 1750Hz plus lesser amounts of
odd-order harmonics. If you feed this to your rig's micro-
phone line (not direct to the transmitter's modulator circuit-
ry), then audio frequencies above 3KHz are very effectively
filtered out, leaving a drift free 1750Hz sine wave as the res-
ultant modulated transmitter output. The series 100K resi-
stor in the tone output line acts as a simple level control and
microphone line decoupler.
Depending upon your set's audio circuitry you may need to
vary the value of this (possibly use a 470K preset instead)
and also maybe add a DC blocking capacitor, eg 100nF, to
prevent any DC voltage offset problems if a direct and non-
decoupled electret microphone (rather than a normal dyn-
amic microphone) is used.
The supply voltage may be taken directly from the rig's
main power supply rail if required, via a momentary push-
to-make button to provide a 1750Hz tone for as long as the
button (together with the rig's PTT) is pressed.
By soldering the components directly to the IC pins you'll
find it fits into the tiniest of spaces in your rig.